Separation of gases via carbonized vinylidene chloride copolymer gas separation membranes and processes therefor

ABSTRACT

A carbonized PVDC copolymer useful for the separation of an olefin from its corresponding paraffin may be made by heating a polyvinylidene chloride copolymer film or hollow fiber having a thickness of 1 micrometer to 20 micrometers to a pretreatment temperature of 100° C. to 180° C. to form a pretreated polyvinylidene chloride copolymer film and then heating the pretreated polyvinylidene chloride copolymer film to a maximum pyrolysis temperature from 350° C. to 750° C. A process for separating an olefin from its corresponding paraffin in a gas mixture is comprised of flowing the gas mixture through the aforementioned carbonized polyvinylidene chloride (PVDC) copolymer to produce a permeate first stream having an increased concentration of the olefin and a second retentate stream having an increased concentration of its corresponding paraffin.

The present invention relates to the field of gas separation using acarbon membrane. More particularly, it relates to the separation ofgases and in particular olefins from their corresponding paraffins froma gas mixture by passing the gas mixture through a carbonized vinylidenechloride copolymer membrane (film or hollow fiber).

Carbon molecular sieves (CMS) and CMS membranes have been used toseparate gases. CMSs may be prepared from a variety of resins that arepyrolyzed at various temperatures and/or under various conditions. Thepyrolysis reduces the resins to carbon, but maintains at least someporosity, in the form of micropores, in the pyrolyzed product. The CMSsthus formed may then be employed in conventional gas separationsequipment employing adsorption of particular gases, such as packed beds,columns, and the like, where the micropore size determines which gas ina gas mixture is adsorbed and which is not. Adsorption and desorptiontechniques may be alternated to carry out the separation, according to,for example, conventional pressure swing or temperature swing adsorptionmethods. CMS membranes have also been used to separate gases by flowinggas mixtures through the CMS membranes.

However, there is a particular challenge in the art to prepare CMSshaving micropores of the correct size(s) for certain particularseparations. Since the use of CMSs to accomplish separations assumesthat the micropores are at least as large as, or larger than, thespecified molecule that will enter the micropores, it is necessary toknow the “size” of the molecule. Different ways to determine thatmolecular size have been developed. One commonly employed approach hasbeen to determine a given molecule's “kinetic diameter.” A referencelisting a variety of these kinetic diameters, based upon their use inzeolite applications, is D. W. Breck, Zeolite Molecular Sieves:Structure, Chemistry and Use, John Wiley & Sons, Inc. (New York, N.Y.1974), 636, and these determinations are frequently used even withrespect to non-zeolite, carbon molecular sieves that are known to haveslit-shaped pores. In view of the above and for purposes hereof, then,the following kinetic diameters, taken from the Breck reference citedsupra, are used herein as the representative molecular diameters for thefollowing molecules: He (2.6 Angstroms, A), H₂ (2.89 Å), N₂ (3.64 Å),CO₂ (3.3 Å), CH₄ (3.8 Å), C₂H₄ (3.9 Å), C₃H₈ (4.3 Å), i-C₄H₁₀ (5.0 Å),SF₆ (sulfur hexafluoride) (5.5 Å), and i-C₈H₁₈ (iso-octane) (6.2 Å).However, because that reference table lacks a kinetic diameter forethane, and the kinetic diameter given therein for propylene is believedby at least some researchers to be inaccurate for CMS materials per se,the Lennard-Jones collision diameters are used herein, instead of theBreck kinetic diameters, for those two materials. These Lennard-Jonescollision diameters are, respectively, C₂H₆ (4.1 Å), and C₃H₆ (4.0 Å).See, for example, Staudt-Bickel C., Koros W. J., “Olefin/paraffin gasseparations with 6FDA-based polyimide membranes,” J. Membr. Sci. (2000)170 (2), 205-214 for further discussion. The kinetic diameters andLennard-Jones collision diameters are referred to together as“representative molecular diameters.”

Polyvinylidine chloride (PVDC) copolymers have been pyrolyzed to formcarbon molecular sieves, but they have tended to form larger pores.Lamond T. G., et al., “6 Å molecular sieve properties of SARAN-typecarbons,” Carbon (1965) 3, 59-63. This article describes preparation ofa CMS, from a polyvinylidene chloride (PVDC) copolymer, that rejectsneopentane (6.0 Å) molecules, but adsorbs smaller molecules, such as, innon-limiting example, CO₂, butane, and iso-butane, non-selectively. Inview of this the authors of that article concluded that their CMS had 6Å micropores.

Another example is disclosed in T. A. Centeno., et al., “Molecular sievegas separation membranes based on poly(vinylidene chloride-co-vinylchloride),” Carbon (2000) 38, 1067-1073. This article describespreparation of a composite carbon membrane using the aforesaid material.The membrane is formed with a thin microporous carbon layer (thicknessof 0.8 micrometers, μm) obtained by pyrolysis of the polymeric film,supported over a macroporous carbon substrate (pore size 1 μm;macroporosity 30 percent, %). Single gas permeation experiments includehelium (He), CO₂, oxygen (O₂), nitrogen (N₂) and methane (CH₄).Selectivities are described as particularly high for O₂/N₂ systems,i.e., a selectivity of about 14 at 25 degrees Celsius (° C.). From thisinformation it can be inferred that the micropore size falls somewherein a range from the representative molecular diameter of 02 (3.46 Å) tothat of N₂ (3.64 Å). This CMS membrane is prepared by pre-treating thesupported film at 200° C., a temperature at which the PVDC copolymerprecursor is melted before carbonization. The fact that melting isrequired means that the disclosed CMS structures cannot be prepared inunsupported forms.

In other research, including for example, Laredo G. C., Meneses E.,Castillo J., Marroquin J. O., Jimeenez-Cruz F., “Adsorption equilibriumand kinetics of branched octane isomers on a polyvinylidenechloride-based carbon molecular sieve,” Energy Fuels (2008) 22 (4)2641-2648, polyvinylidene chloride copolymer-based CMSs have beenprepared that exhibit relatively large micropore sizes and pore volumesthat are suitable for separation of correspondingly large molecules,i.e., those having a representative molecular diameter greater than 5.0Å.

More recently, WO/2016/003680 described forming a CMS from PVDCcopolymers using a two-step pyrolysis at high temperatures from 800° C.to 1700° C. The CMS formed had an average pore size in the range of 3 Åto 5 Å. These CMS were described as being useful for separatingPropylene (C₃H₆) and propane (C₃H₈); carbon dioxide (CO₂) and nitrogen(N₂); N₂ and methane (CH₄); ethylene (C₂H₄) and ethane (C₂H₆); andn-butane (C₄H₁₀) and i-butane (C₄H₁₀).

CMS membranes formed from PVDC copolymers have also been made, but theyhave suffered from low or reverse (rejection of hydrogen with presenceof hydrocarbons) hydrogen selectivity as described by M. B. Rao and S.Sircar in J. Membrane Science, 85 (1993) 253-264; T. A. Centeno and A. BFuertes in Carbon, 38 (2000) 1067-1073 and K. Zhang and J. D. Way in J.Membrane Science 369(2011)243-249.

The gas permeation properties of a membrane can be determined by gaspermeation experiments. Two intrinsic properties have utility inevaluating separation performance of a membrane material: its“permeability,” a measure of the membrane's intrinsic productivity; andits “selectivity,” a measure of the membrane's separation efficiency.One typically determines “permeability” in Barrer (1 Barrer=10⁻¹⁰ [cm³(STP) cm]/[cm² s cmHg], calculated as the flux (n_(i)) divided by thepartial pressure difference between the membrane upstream and downstream(Δp_(i)), and multiplied by the thickness of the membrane (l).

$P_{i} = \frac{n_{i}\mspace{14mu} l}{\Delta \; p_{i}}$

Another term, “permeance,” is defined herein as productivity of themembrane or hollow fiber membranes and is typically measured in GasPermeation Units (GPU) (1 GPU=10⁻⁶ [cm³ (STP)]/[cm² s cmHg]), determinedby dividing permeability by effective membrane separation layerthickness.

$\left( \frac{P_{i}}{l} \right) = \frac{n_{i}}{\Delta \; p_{i}}$

Finally, “selectivity” is defined herein as the ratio of one gas'spermeability through the membrane or permeance relative to the sameproperty of another gas. It is measured as a unitless ratio.

$\propto_{i\text{/}j}{= {\frac{P_{i}}{P_{j}} = \frac{\left( {P_{i}\text{/}l} \right)}{\left( {P_{j}\text{/}l} \right)}}}$

Thus, it would be desirable to realize a CMS membrane and process tomake the CMS membrane from PVDC that would be useful in separatingolefins from the corresponding paraffins from gas mixtures such as thoseencountered in syngas, gases generated in oil refineries, natural gasand olefin cracker gas streams. It would be particularly desirable toprovide a PVDC CMS that is in the form of an un-supported membrane orhollow fiber.

One aspect of the invention is a method of making a carbonizedpolyvinylidene chloride copolymer useful to separate an olefin from itscorresponding paraffin comprising,

(a) providing a polyvinylidene chloride copolymer film or hollow fiberhaving a thickness of 1 micrometer to 20 micrometers,

(b) heating the polyvinylidene chloride copolymer film to a pretreatmenttemperature of 100° C. to 180° C. to form a pretreated polyvinylidenechloride copolymer film, and

(c) heating the pretreated polyvinylidene chloride copolymer film to amaximum pyrolysis temperature from 350° C. to 750° C.

A second aspect of the invention is a process for separating an olefinfrom a gas mixture having the olefin's corresponding paraffin, themethod comprising

-   -   (i) providing the carbonized polyvinylidene chloride copolymer        membrane of any one of the preceding Claims and    -   (ii) flowing the gas mixture through said carbonized        polyvinylidene chloride copolymer membrane to produce a permeate        first stream having an increased concentration of the olefin and        a second retentate stream having an increased concentration of        its corresponding paraffin.

It has been discovered that the separation of olefins (e.g., ethyleneand propylene) from their corresponding paraffins (e.g., ethane andpropane), may be achieved by flowing these gases through carbon filmsmade by the aforementioned process, but only when the film isparticularly thin. It is unknown why, but without being limiting in anyway, it is believed that the particular process may realize anasymmetric microstructure across the film thickness where there may belarge pores at the surface of the film and a narrow band within the filmthat has a smaller pore size. It has been discovered that the thicknessis critical. This may be due to how the HCl evolves when forming thecarbonized PVDC. The performance of the film which may be due to aparticular assymetry may be a result of varying localized atmospheresduring the pre-treatment and pyrolysis (e.g., partial pressure of HCl),which may be due to the thickness of the films, restraining of the filmsor combinations thereof. The thickness of the film should be at least 1micrometer thick to 20 micrometers thick. If the thickness is less than1 micrometer the separation or integrity of the film tends toinsufficient. If the thickness is greater than about 20 micrometers thepermeance suffers. Desirably, the thickness is from about 2 or 5micrometers to 15 or 12 micrometers.

The PVDC CMSs of the invention may be prepared from a vinylidenechloride copolymer, comprising a vinylidene chloride monomer and atleast one additional comonomer. The comonomer may be selected from avariety of materials, including in particular embodiments a vinylmonomer, vinyl chloride monomer, an acrylate monomer, a methacrylatemonomer, a styrenic monomer, acrylonitrile, methacrylonitrile, itaconicacid, chlorotrifluoroethylene, and combinations thereof. In moreparticular embodiments examples of the vinyl monomers include vinylchloride, vinyl acetate, acrylonitrile, and combinations thereof. Moreparticular examples of the acrylate monomers include methyl acrylate,ethyl acrylate, butyl acrylate, and combinations thereof. Moreparticular examples of methacrylate monomers include methylmethacrylate, butyl methacrylate, and combinations thereof. A moreparticular example of styrenic monomers is styrene itself.

In proportion it is preferred that the vinylidene chloride basedcopolymer, which is herein termed a polyvinylidene copolymer (PVDC),includes at least 60 wt % of vinylidene chloride, based on total weightof the copolymer, and in more preferred embodiments at least 70 wt %.However, it is further desired that the PVDC contains a maximum of 97 wt% vinylidene chloride, and thus preferably contains a minimum of atleast 3 wt % of the comonomer or comonomer combination; more preferablyfrom 3 wt % to 40 wt %; still more preferably from 3 wt % to 30 wt %;and most preferably from 3 wt % to 20 wt %.

Particular embodiments of PVDCs that are suitable for use in theinvention are those including as a comonomer an acrylate, such as methylacrylate, ethyl acrylate, butyl acrylate, or a combination thereof, inan amount from 3 wt % to 20 wt %, based on the weight of the PVDC as awhole; more preferably from 3.5 wt % to 15 wt %; and most preferablyfrom 4 wt % to 12 wt %. Another particular embodiment is a PVDCincluding vinyl chloride in an amount from 3 wt % to 30 wt %; morepreferably from 7 wt % to 28 wt %; and most preferably from 9 wt % to 25wt %.

It is also preferred that the overall weight average molecular weight(Mw) of the PVDC copolymer ranges from 10,000 to 250,000; morepreferably from 50,000 to 200,000; and most preferably from 60,000 to150,000.

Use of additives in the PVDC is also contemplated as being within thescope of the invention. Common additives may include, but are notnecessarily limited to, epoxidized oil stabilizers such as epoxidizedsoybean oil, epoxidized linseed oil, and the diglycidyl ether ofbisphenol A. Also frequently employed are liquid plasticizers such asaliphatic and aromatic esters, including for example dibutyl sebacate,acetyl tributyl citrate, dioctyl phthalate, and the like, andcombinations thereof. Other common additives may include lubricants,such as polyethylene wax, paraffin wax, oxidized polyethylene wax, andcombinations thereof. Lubricants may optionally be included, and maycomprise, for example, high density polyethylene, acrylate copolymersand silicone polymers, and combinations thereof. Another group ofadditives that may be included are acid scavengers such as epoxycompounds, magnesium hydroxide, magnesium oxide, tetrasodiumpyrophosphate, calcium phosphate, magnesium phosphate, DHT 4A (asynthetic hydrotalcite-like halogen scavenger available from KyowaChemical Industry), calcium oxide, calcium carbonate, and combinationsthereof. Antioxidants such as phenolics may also be incorporated.Combinations of any or all of these types of additives may be includedin the PVDC.

In proportion, it is preferred that the total amount of all additivescombined be no more than 15 wt %, and more preferably no more than 8 wt% or 3 wt %. In many applications, however, an amount of all additivescombined of at least 2 wt % may be typical, with use thereof thereforeranging preferably from 2 wt % to 8 wt %, and more preferably from 2 wt% to 3 wt %. Those skilled in the art will be aware of the use of suchadditives and their indications and contraindications without furtherdirection herein.

Those skilled in the art will also be aware of a variety of means andmethods for preparing copolymers. However, in general any of the typicalor conventional methods of polymerization, including but not limited tomass polymerization, suspension polymerization, and emulsionpolymerization, and preferably suspension polymerization or emulsionpolymerization, may be employed. It is generally preferred thatpolymerization is carried out at a temperature that ensures avoidance ofdegradation of all of the PVDC components, e.g., preferably from 10° C.to 120° C.; more preferably from 20° C. to 100° C.; and most preferablyfrom 30° C. to 90° C.

Following completion of the copolymerization, the PVDC may be formedinto a film or hollow fiber by any suitable method such as those knownin the art. For example the PVDC may be melt-extruded, solution or latexcast in order to form the PVDC into a thin film or hollow fiber. Wherefilms are desired, a conventionally known preparation process such as ablown film process, for example, a double bubble process or a cast filmtentering process, may be especially useful to produce a biaxiallyoriented film. It is more preferred that a double bubble process beemployed in order to concurrently extrude, biaxially orient, and annealthe PVDC film. Fibers may be produced by uniaxial stretching using knownfiber processes for PVDC copolymers, and may be round or shaped hollowfibers, or of any other desired hollow fiber morphology. It is alsocontemplated that precursor films and/or fibers may be coextruded withmultiple PVDC copolymers and/or with other polymers.

It is noted that either the film or fiber preparation process mayoptionally include stretching, such as stretching of the resin to form amelt-extruded film or fiber. This stretching may, in particularembodiments, be particularly effective in inducing more rapidcrystallization and in increasing, and therefore improving, alignment ofthe PVDC crystallites. Desirably the stretch ratio ranges from 1 to 8,more desirably from 1 to 6, still more desirably from 1 to 4, and mostdesirably from 2 to 4.

Generally it is useful for the PVDC to have some amount ofcrystallinity. In the present invention this crystallinity typicallyranges from 25% to 75% of the resin or formed film, as measured bydifferential scanning calorimetry (DSC) according to ASTM D3418. It ismore preferred that this level ranges from 30% to 55%, and mostpreferred that this level ranges from 35% to 50%. Thus, inclusion of acomonomer generally helps to reduce precursor crystallinity to ensurethe desired range, and also to reduce the melt temperature and therebyimprove processability of the resulting copolymer. In general, inclusionof bulkier monomers may tend to reduce overall copolymer crystallinityby a greater amount than inclusion of less bulky monomers. Thus, forexample, butyl acrylate will tend to reduce crystallinity more than, forexample, methyl acrylate or ethyl acrylate, assuming such is/are used inthe same mole percent (mol %) based on final copolymer composition.

To form the PVDC CMS films or hollow fibers of the present invention apre-treatment prior to pyrolysis is employed. Generally thepre-treatment is used to stabilize, or “lock,” the copolymer structureprior to carbonization thereof. In this step the PVDC film or fiber areheated, below the melting temperature thereof (typically less than about180° C., depending upon the exact composition of the precursor), inorder to dehydrochlorinate the film to the extent of at least 10%. Asused herein, the term “at least 10% dehydrochlorinated” means that thefilm or fiber has been pre-treated, by removing hydrogen chloride, to apoint at which the PVDC copolymer film or fiber no longer melts and, infact, begins to become infusible. It is well-accepted in the art thatsuch a change in molecular kinetics begins to occur at a point ofapproximately 10% dehydrochlorination and is completed or maintained asthe level of dehydrochlorination increases above that point. This stepis termed a “pre-treatment” because it occurs prior to a pyrolysis step,which is the treatment step wherein carbonization is accomplished.

During the pre-treatment the copolymer structure's temperature ispreferably maintained in a range of from 100° C. to 180° C., morepreferably from 120° C. to 160° C., and most preferably from 130° C. to150° C. This is preferably done in air for convenience, but otheratmospheres, such as N₂ and other inert gases or oxidizing gases such asCO₂, or combinations thereof, may also or alternatively be used, sincegenerally only minor levels of oxidation of the copolymer areanticipated within the overall given temperature range. Achievement ofthe desired dehydrochlorination, that is responsible for the formationof the locked structure, may be accomplished by exposure to a source ofhigh energy irradiation, such as gamma rays, an electron beam,ultraviolet light, or a combination thereof. The time may vary from 1hour (hr) to 48 hr, preferably from 1 hr to 24 hr, and most preferablyfrom 1 hr to 12 hr, as needed to reach the at least 10%dehydrochlorination point, at which the copolymer begins to becomeinfusible, i.e., no longer able to be melted. The dehydrochlorinationdegree can vary from 5% to 100%, depending upon pretreatment temperatureand time. Where more than visual confirmation of the beginning ofinfusibility is desired, additional confirmation of the percentage ofdehydrochlorination may be obtained by means of, for example, ThermoGravimetric Analysis (TGA), using standard and well-known methods andequipment.

During the pre-treatment the fiber or film may be restrained to maintainits shape and preferably is. The particular restraining method may beany known in the art and may be held in tension or compression. In aparticular embodiment, particularly for films, they are restrained byapplying a compressive force. In particular the film is placed betweentwo flat substrates that may be impervious to gases including the HClbeing removed. Illustratively, the film may be constrained between twolow surface energy plates (e.g., TEFLON plates or sheets), which arefurther interposed between two metal, ceramic or graphite plates.Alternatively, the plates may be pervious to gases such as the HCl beingremoved such as honeycomb structures. The amount of tension orcompression may be any useful amount, but typically may range from 0.01MPa to 10 MPa, from 0.1 to 1 MPa, or from 0.1 to 0.5 MPa. In the samemanner, the restraining during pyrolysis may be performed in the samefashion with similar substrates, which can withstand the maximumpyrolysis temperatures used.

Following the dehydrochlorination pre-treatment, the pre-treated film orpre-treated fiber, or alternatively pre-treated CMS material, ispyrolyzed. Preferably such pyrolysis results in at least 90 wt % of thecopolymer becoming carbonized, more preferably at least 95 wt %, andmost preferably at least 99 wt %. As already pointed out hereinabove,this pyrolysis is also termed “carbonization,” because the resultthereof is that the copolymer is converted to the carbon-only, or nearcarbon-only, skeleton of its copolymer structure, i.e., all or virtuallyall atoms other than carbon have been removed, but the carbon-carbonbonds remain substantially intact, and the CMS may now be termed to be“carbonaceous.” The pyrolysis may be carried out using any meansgenerally known to those skilled in the art, but may be carried out atan attained maximum temperature within the range of from 350° C. to 750°C. Desirably, the temperature is at least 400° C., 450° C. to at most700° C., 650° C., 600° C. or 550° C.

The method may form a PVDC CMS membrane that has a combination of olefinpermeance and olefin/paraffin selectivity highly useful for separatingthe olefin from gases containing its corresponding paraffin. Suchcombinations generally require reasonably high olefin permeance withreasonably high selectivity or lower olefin permeance with higherselectivity.

Surprisingly, the PVDC membranes may have an average pore size that islarger than olefin desired to be separated (e.g., ethylene or propylene)and a larger gas molecule than olefin (e.g., ethane or propane) in a gasmixture. The larger gas molecule may be comprised of even larger olefins(e.g., butylene) and paraffins (e.g., butane). It is surprising, sincethe selectivity of ethylene/ethane or propylene/propane may becommercially useful, even though the average pore size of the membraneis larger than the larger gas molecule diameter, which would indicatethat such larger gas molecule would not be preferentially rejected whenflowing through the membrane (i.e., fits into the pores and would beexpected to flow through the membrane). Thus, this gives rise to thebelief an asymmetric structure is formed in the film as described above.In general, the average pore size of the PVDC CMS membrane at least 3 Å,4 Å or even 5 Å to at most about 15 Å. The average pore size of themembrane may be determined by adsorption.

In addition to average micropore size, it is also often desirable in theart to optimize total micropore volume, which may be measured via theBrunauer-Emmett-Teller (BET) method at liquid N₂ temperature. Such maybe further confirmed via helium (He) pycnometry and mercury (Hg)intrusion. For most separations applications, a total micropore volumeof at least 0.10 mL/g, preferably at least 0.15 mL/g, more preferably atleast 0.20 mL/g, according to the BET method at liquid N₂ temperature,is needed to ensure commercially efficient desirable gas adsorption.

The average micropore size and/or average micropore volume seem tosuffer little, if any, alteration when additional factors, including butnot limited to ramp rate to reach the attained maximum pyrolysistemperature, and/or hold time at the attained maximum pyrolysistemperature, are introduced and/or considered. For example, forindustrial purposes, ramp rates ranging from 0.1° C./min to 10° C./minare typical, and hold times may range from 0 minutes (min) (i.e.,ramping to the attained maximum temperature followed by immediate activeor passive temperature reduction) up to 60 min (i.e., holding at theattained maximum pyrolysis temperature for up to 60 min prior to activeor passive temperature reduction) are typical. The atmosphere may be anythat realizes the PVDC CMS membrane. That is the atmosphere that doesnot substantially oxidize the PVDC and may include inert, or reducingatmospheres such as static or flowing nitrogen, inert gas (e.g., argon),carbon monoxide, hydrogen or any combination thereof.

The CMS membranes are particularly suitable for separating ethylene orpropylene (olefin) in a gas feed containing its corresponding paraffin(e.g., ethane or propane) or one or more other larger gas molecules. Inperforming the process, the gas feed is flowed (over and through themembrane) such that a first stream (permeate) having an increasedconcentration of the desired olefin and second stream (retentate) havingan increased concentration of the other gas molecule(s) including thedesired olefin's corresponding paraffin results. When practicing theprocess, the CMS membrane is generally fabricated into a modulecomprising a sealable enclosure comprised of one or more of the carbonmolecular sieve membranes described herein contained within the sealableenclosure. The sealable enclosure has an inlet for introducing a gasfeed comprised of olefin and at least one other larger gas molecule; afirst outlet for permitting egress of a permeate gas stream (i.e.,olefin); and a second outlet for egress of a retentate gas stream (i.e.,the other larger gas molecule(s)).

EXAMPLES PVDC Copolymer Film Preparation: Melt Extruded Films of MA 4.8wt % Copolymer

Base PVDC copolymer with 4.8 wt % methyl acrylate (MA) comonomer (namedas MA4.8 wt %, Mw=96,000, The Dow Chemical Company, Midland, Mich.) wasblended with 2 wt % epoxidized soybean oil (based on total amount ofblend), 4 wt % dibutyl sebacate, and 2 wt % PLASTISTRENGTH L-1000 anacrylic lubricant available from Arkema PLC, France. The blend wasextruded through a 1.75 inch width film die (controlled at 174° C.)followed by water quench and stretch winding. The wind rate wascontrolled to obtain films of 2 mil (1 mil=25.4 micrometer). The filmsafter winding were cut into approximately 12 inch wide and 2 feet lengthpieces and laid on flat desktop for about one week. Coupons of ⅞ inchdiameter were cut for carbonization as described below.

Melt Extruded Films of MA 8.5 wt % Copolymer

PVDC copolymer with 8.5 wt % methyl acrylate comonomer (named as MA8.5wt %, Mw=85,000, The Dow Chemical Company) was blended with 2 wt %epoxidized soybean oil and 2 wt % PLASTISTRENGTH L-1000. The blend wasextruded in the same manner as above. The wind rate was controlled toobtain films that are 2 mil (˜50 micrometers) thick. The films afterwinding were cut into approximately 12 inch wide and 2 feet lengthpieces and laid on flat desktop for about one week.

Melt Bubble Extruded Films of MA 4.8 wt % Copolymer

Base PVDC copolymer with 4.8 wt % methyl acrylate (MA) comonomer (namedas MA4.8 wt %, Mw=96,000, The Dow Chemical Company, Midland, Mich.) wasblended with 2 wt % epoxidized soybean oil (based on total amount ofblend), 4 wt % dibutyl sebacate, and 2 wt % PLASTISTRENGTH L-1000 anacrylic lubricant available from Arkema PLC, France. The resin was meltextruded using a commercial bubble extruder to form films having athickness of 0.4 mil (˜10 micrometers).

Melt Bubble Extruded Films of VC17.6 wt % Copolymer

PVDC copolymer with 17.6 wt % vinyl chloride (VC) comonomer (named asVC17.6 wt %, The Dow Chemical Company) was blended with 2 wt %epoxidized soybean oil and 2 wt % PLASTISTRENGTH L-1000. The resin wasmelt extruded using a commercial bubble extruder to form films having athickness of 0.4 mil (˜10 micrometers).

TABLE 1 Precursor Films Precursor Film thickness Film # Preparationmethod Base resin [mil] 1 Melt extrusion MA4.8 wt % 2 2 Melt extrusionMA8.5 wt % 2 3 Bubble Melt extrusion MA4.8 wt % 0.4 4 Bubble Meltextrusion VC17.6 wt % 0.4

Carbon Membrane Formation

A two-step pyrolysis approach was used. The precursor films were heatedto a first temperature of 130-150° C. for 24 hours in a low temperatureoven purged by 2 L/min of air (pretreated films), which was followed byfurther heating to pyrolyze the pretreated films to temperatures in therange of 350-950° C. in a 6″ ID quartz tube furnace purged by 5 L/min ofnitrogen.

For the initial low temperature pretreatment, 12 disks (⅞ inch diameter)sandwiched between porous ceramic honeycomb plates, through which HClgenerated should be transported out swiftly. A scrubber connected tothis oven contained a 10 wt % sodium hydroxide aqueous solution. Aloaded oven was heated at 1° C./min to the temperature shown in Table 2(Pretreatment Temperature) and held for 24 hours under 2 L/min of airpurge.

For the second heating step, the 12 pretreated disks were sandwichedbetween the porous ceramic honeycomb plates were loaded into a 6″ IDquartz tube furnace. A scrubber connected to this furnace contained a 10wt % sodium hydroxide aqueous solution. The furnace was raised to 500°C. at 3° C./min), and held for 30 minutes at the final temperature andthen cooled down to room temperature (˜25° C.). After cooling down, thecarbon membranes were put into a storage box continuously purged withdry nitrogen at a flow rate of 5 Liter/min.

Carbon Membrane Permeation Test

The carbon membranes were masked onto a standard 25 mm filter holder(Millipore #4502500, EMD Millipore Corp., Germany) using an impermeablealuminum tape, leaving an open defined permeation area. A two-part epoxy(J-B Weld twin tube) was then applied along the interface of the tapeand the carbon membranes. Mixed permeation tests of several gas specieswere conducted at 20° C. with a continuous upstream feed of either anequimolar mixture of H₂/CO₂/CH₄ (total 75 sccm, 1 atm) or an equimolarmixture of C₂H₄/C₂H₆/C₃H₆/C₃H₈ (total 50 sccm, 1 atm), and downstream Hepurge (2.0 sccm, 1 atm). The permeate carried by the He purge gas wasanalyzed by a GC (gas chromatograph) with a TCD (thermal conductivitydetector for H₂ and CO₂) and FID (flame ionization detector for all thehydrocarbons). The concentrations in all gases were lower than 5%, sothe gas flow rate in downstream was considered the same as the He flowrate. The membrane permeate rate was calculated using the He purge flowrate times the permeate concentrations measured by GC. The tests wererun for several hours to days until the permeate concentrations weresteady. The parameters to make the carbon membranes are shown in Table2. The resulting permeation results are shown in Table 3.

TABLE 2 CMS film Pre-treatment Temp. Final Pyrolysis Temp. PrecursorExample [° C.] [° C.] * film # Ex. 1 130 500 3 Ex. 2 130 500 4 Comp. Ex1 130 500 1 Comp. Ex. 2 130 500 2

TABLE 3 Ethylene/ Propylene/ Hydrogen Methane Ethylene Ethane PropylenePropane ethane propane CMS film Permeance Permeance H₂/CH₄ PermeancePermeance Permeance Permeance selectivity selectivity Ex. (GPU) (GPU)Selectivity (GPU) (GPU) (GPU) (GPU) [—] [—] Ex. 1 132 1.76 75 3.30 0.922.14 0.09 3.6 23.5 Ex. 2 133 1.47 90 2.66 0.70 1.43 0.03 3.8 51.1 Comp.Ex. 1 96 0.28 343 0.59 0.10 0.24 BDL 6.1 N/A Comp. Ex. 2 83 0.53 1561.05 0.17 0.42 BDL 6.3 N/A

Comp. Ex.1 and Comp. Ex.2 are highly selective CMS membrane for H₂/CH₄separation. However, the flux (permeance) of hydrocarbon molecules aretoo small, particularly ethylene and propylene which have a permeancelower than ˜1GPU, making them not useful industrially.

Ex.1 and Ex.2 were made essentially the same as Comp. Ex. 1 and Ex. 2respectively but were much thinner (˜50 micrometers versus ˜10micrometers). The thinner CMS films of Ex.1 and Ex.2 had substantiallyworse H₂/CH₄ selectivities, making them unattractive for thatapplication anymore. However, the fluxes of ethylene and propyleneincreased to industrially useful ranges (>1 GPU at 20° C.) forolefin/paraffin separations and displayed useful olefin/paraffinselectivities.

1. A method of making a carbonized polyvinylidene chloride copolymeruseful to separate an olefin from its corresponding paraffin comprising,(a) providing a polyvinylidene chloride copolymer film or hollow fiberhaving a thickness of 1 micrometer to 20 micrometers, (b) heating thepolyvinylidene chloride copolymer film to a pretreatment temperature of100° C. to 180° C. to form a pretreated polyvinylidene chloridecopolymer film, and (c) heating the pretreated polyvinylidene chloridecopolymer film to a maximum pyrolysis temperature from 350° C. to 750°C.
 2. The method of claim 1, wherein the film or fiber is restrained insteps (b) and (c) by applying a force.
 3. The method of claim 1, whereinthe maximum pyrolysis temperature is at most 650° C.
 4. The method ofclaim 1, wherein the polyvinylidene chloride copolymer film is apolyvinylidene chloride copolymer comprised of vinylidene chloride andat least one of the following: a vinyl monomer; a vinyl chloridemonomer; an acrylate monomer; a methacrylate monomer; a styrenicmonomer; acrylonitrile, methacrylonitrile; itaconic acid;chlorotrifluoroethylene that have been copolymerized.
 5. The method ofclaim 1, wherein the thickness is from 1 micrometers to 10 micrometers.6. The method of claim 1, wherein the polyvinylidene chloride copolymerfilm is formed by melt-extrusion at a stretch ratio from 1 to
 8. 7. Aprocess for separating an olefin from a gas mixture having the olefin'scorresponding paraffin, the method comprising (i) providing thecarbonized polyvinylidene chloride copolymer membrane of claim 1 and(ii) flowing the gas mixture through said carbonized polyvinylidenechloride copolymer membrane to produce a permeate first stream having anincreased concentration of the olefin and a second retentate streamhaving an increased concentration of its corresponding paraffin.
 8. Theprocess of claim 7, wherein the olefin is ethylene or propylene.
 9. Theprocess of claim 8, wherein the paraffin is ethane or propane.
 10. Theprocess of claim 7, wherein the polyvinylidene chloride copolymermembrane has an average pore size greater than the olefin or paraffin asdetermined by gas permeation employing gas probe molecules of differingsizes.
 11. The process of claim 10, wherein the average pore size isgreater than 4 angstroms.
 12. The process of claim 11, wherein theaverage pore size is greater than 5 angstroms.